It is conventional, as disclosed in JP-U-2-132515 for example, to provide an ultrasonic sensor on a vehicle door and detect a distance from the ultrasonic sensor to an obstacle so that the vehicle door may be prevented from touching the obstacle when it is opened.
In case of detecting the distance from the vehicle door to the obstacle by generating an ultrasonic wave pulse in a direction perpendicular to the outer surface of the vehicle door from the ultrasonic sensor provided on the vehicle door and receiving the ultrasonic wave pulse reflected by the obstacle, the area, which one ultrasonic sensor can cover in detecting an obstacle, is not sufficient relative to the size of the vehicle door.
According to the conventional system, therefore, a plurality of ultrasonic sensors is provided on one vehicle door to detect an obstacle over a wide area of the vehicle door. However, such a number of sensors necessarily increase total system costs to a large extent.
It is therefore proposed (Japanese patent application No. 2008-246665) to detect an obstacle, which a vehicle door will possibly touch, over almost all surface area of a vehicle door by a single sensor.
In this vehicle door opening angle control system, a laser sensor is provided on a vehicle door near a vehicle door pivot axis. This laser sensor emits laser light to scan a plane, which is deviated a predetermined angle in a direction of opening of the vehicle door. If an obstacle is present within the scanned plane, the laser light is reflected by such an obstacle and received by the laser sensor. It is thus made possible to always detect an obstacle, which is present ahead of the vehicle door by the predetermined angle, when the vehicle door is opened.
However, if a subject vehicle is parked closely in parallel to the other vehicle for example, the laser light projected from the laser sensor becomes incident to the side surface of the other vehicle with a shallow angle of incidence. When the incident laser light is reflected and scattered by the side surface of the other vehicle, a large part of reflected or scattered laser light travels in directions different from the direction toward the laser sensor. The laser sensor thus receives only a small part of the reflected laser light. As a result, it becomes impossible to detect the other vehicle (distance thereto), which is an obstacle, although the other vehicle actually is present. Further, a similar situation, in which a sufficient amount of reflected laser light cannot be received from an obstacle, will arise, if a reflective body is in black or similar color and its reflectivity of the laser light is low.